Several standard protocols for wireless local area networks, commonly referred to as WLANs, are becoming popular. These include protocols such as 802.11 (as set forth in the 802.11 wireless standards), home RF, and Bluetooth. The standard wireless protocol with the most commercial success to date is the 802.11b protocol although successors such as next generation protocols, such as 802.11g, are also gaining popularity.
While the specifications of products utilizing the above standard wireless protocols commonly indicate data rates on the order of, for example, 11 MBPS and ranges on the order of, for example, 100 meters, these performance levels are rarely, if ever, realized. Performance shortcomings between actual and specified performance levels have many causes including attenuation of the radiation paths of RF signals, which are typically in the range of 2.4 GHz in an operating environment such as an indoor environment. Base to receiver ranges are generally less than the coverage range required in a typical home, and may be as little as 10 to 15 meters. Further, in structures having split floor plans, such as ranch style or two story homes, or those constructed of materials capable of attenuating RF signals, areas in which wireless coverage is needed may be physically separated by distances outside of the range of, for example, an 802.11 protocol based system. Attenuation problems may be exacerbated in the presence of interference in the operating band, such as interference from other 2.4 GHz devices or wideband interference with in-band energy. Still further, data rates of devices operating using the above standard wireless protocols are dependent on signal strength. As distances in the area of coverage increase, wireless system performance typically decreases. Lastly, the structure of the protocols themselves may affect the operational range.
Repeaters are commonly used in the mobile wireless industry to increase the range of wireless systems. However, problems and complications arise in that system receivers and transmitters may operate at the same frequency in a WLAN utilizing, for example, 802.11 or 802.16 WLAN wireless protocol. In such systems, when multiple transmitters operate simultaneously, as would be the case in repeater operation, difficulties arise. Typical WLAN protocols provide no defined receive and transmit periods and, thus, because random packets from each wireless network node are spontaneously generated and transmitted and are not temporally predictable, packet collisions may occur. Some remedies exist to address such difficulties, such as, for example, collision avoidance and random back-off protocols, which are used to avoid two or more nodes transmitting packets at the same time. Under 802.11 standard protocol, for example, a distributed coordination function (DCF) may be used for collision avoidance.
Such operation is significantly different than the operation of many other cellular repeater systems, such as those systems based on IS-136, IS-95 or IS-2000 standards, where the receive and transmit bands are separated by a deplexing frequency offset. Frequency division duplexing or multiplexing (FDD or FDM) operation simplifies repeater operation since conflicts associated with repeater operation, such as those arising in situations where the receiver and transmitter channels are on the same frequency, are not present.
Other cellular mobile systems separate receive and transmit channels by time rather than by frequency and further utilize scheduled times for specific uplink/downlink transmissions. Such operation is commonly referred to as time division duplexing or multiplexing, such as TDD or TDM. Repeaters for these systems are easily built, as the transmission and reception times are well known and are broadcast by a base station. Receivers and transmitters for these systems may be isolated by any number of means including physical separation, antenna patterns, or polarization isolation.
Thus, WLAN repeaters operating on the same frequencies without TDD or TDM capability have unique constraints due to the above spontaneous transmission capabilities and therefore require a unique solution. Since these repeaters use the same frequency for receive and transmit channels, some form of isolation must exist between the receive and transmit channels of the repeater. While some related systems such as, for example, CDMA systems used in wireless telephony, achieve channel isolation using sophisticated techniques such as directional antennas, physical separation of the receive and transmit antennas, or the like, such techniques are not practical for WLAN repeaters in many operating environments such as in the home where complicated hardware or lengthy cabling is not desirable or may be too costly.
One system, described in International Application No. PCT/US03/16208 and commonly owned by the assignee of the present application, resolves many of the above identified problems by providing a repeater which isolates receive and transmit channels using a frequency detection and translation method. The WLAN repeater described therein allows two WLAN units to communicate by translating packets associated with one device at a first frequency channel to a second frequency channel used by a second device. The direction associated with the translation or conversion, such as from the frequency associated with the first channel to the frequency associated with the second channel, or from the second channel to the first channel, depends upon a real time configuration of the repeater and the WLAN environment. The WLAN repeater may be configured to monitor both channels for transmissions and, when a transmission is detected, translate the received signal at the first frequency to the other channel, where it is transmitted at the second frequency.
The above described approach solves both the isolation issue and the spontaneous transmission problems as described above by monitoring and translating in response to packet transmissions and may further be implemented in a small inexpensive unit. The base concept of the above described approach is generally suited to scenarios where a single repeater is used for example between an Access Point (AP) and a mobile communication unit or station.
However, in multiple repeater environments where, for example, two or more repeaters are used within the same WLAN environment, undesirable interaction such as jamming or feedback may occur between two repeaters. Potential causes include operating two or more repeaters on the same frequencies where the repeaters are providing repeater servicing for clients from the same AP. Such a conflict may exist, for example, where a client device/station (STA) can only be heard by a single repeater transmitting on a first frequency (F1), and the repeater transmits to the AP on a second frequency (F2). Another repeater may also transmit on F1 thus interfering with station transmissions also at F1.
A second example of undesirable interaction may occur when repeaters are chained together in a straight line order, such as AP-R1-R2-STA. In such an exemplary scenario, an AP may transmit, for example, on F1, repeater R1 transmits on F2, and repeater R2 will transmit on F1 again to STA. Problems may arise due to feedback or jamming caused by transmission loop-back arising from a node, for example, an AP or STA, hidden or lower in receive power than the signal from an adjacent repeater, while repeaters R1 and R2 operate on the same pair of channels. Thus for example, if R2 receives a signal transmitted by R1, and re-transmits on the same frequency used by R1 to receive causing either a reduction in signal quality or a constructive feedback situation where each repeater progressively amplifies the signal ultimately resulting in an oscillation.